1. Field of the Invention
The present invention relates to a power supply for a gas discharge lamp, and particularly to such a device employing a switching regulator reflecting a unity power factor and no third harmonic distortion to the ac line, an inverter and a resonant network including the lamp load, the inverter operating at the resonant frequency of the network to provide a sinusoidal output voltage.
2. Description of the Prior Art
Various types of gas discharge lamps are widely used for lighting purposes. These include fluorescent lamps, high intensity discharge lamps of different types including the metal halide varieties and sodium lamps of both high and low pressure. A common feature of all these lamps is that they require some type of ballast for operation. Ordinarily ballast transformers are used. This approach has several shortcomings. For operation at line frequency, the ballast must be of substantial physical size and weight, resulting from the large magnetic transformers and capacitors that are required. Efficiency is low. The ballast must be operated at the rated line voltage, and any serious deviation can cause either the ballast to overheat or the lamp to flicker. Dimming of the lamp is difficult or impossible.
One approach of the prior art to overcome these difficulties has been the use of switching regulators to provide to the lamp a direct current that is switched on and off at a high frequency. While dc will light the lamp adequately, a specially designed lamp is required if the lamp lifetime is not to be sacrificed considerably. Moreover, such circuits require that the ac line voltage first be rectified and filtered for input to the regulator.
If inductive filtering is used, a very poor power factor will result, and the inductor may have to be the same large size as the original ballast. Also, line distortion is created by the combination of the inductor and the bridge rectifier. If capacitive filtering is used, all of the current will be conducted during the peak of the ac line cycle. This produces "third harmonic distortion" which heats up the pole transformers and requires extra heavy wiring between the device and the power source.
Another approach of the prior art is shown in the U.S. Pat. No. 3,999,100. This supply uses a switching regulator in conjunction with a commutator to provide power to a metal halide lamp. The commutator is operated at or near the ac line frequency.
The principle object of the present invention is to provide an improved electronic ballast for a gas discharge lamp. Other objectives are to provide such a lamp power supply wherein:
(a) no third harmonic distortion is produced and the supply has a near unity power factor;
(b) improved efficiency results from supplying the lamp with high frequency ac power;
(c) dimming is facilitated for both fluorescent and high intensity gas discharge lamps;
(d) the ballast will operate properly despite line voltage variation;
(e) all components are lightweight, small in size and inexpensive;
(f) sufficiently high output voltage is provided to start high intensity gas discharge lamps;
(g) circuitry automatically compensates for changing lamp performance characteristics, including during the warm-up period of high intensity lamps;
(h) sinusoidal output voltage is produced, with concomitant benefits in efficiency, less stringent performance requirements and hence lower cost for the circuit switching transistors, and substantial elimination of radio frequency interference;
(i) dimming may be accomplished in response to ambient light conditions; and
(j) provision is made for battery operation in the event of ac power line failure.
Yet another object of the invention is to provide the benefits of (a) (d) (e) for other than ballast applications.